Python Review

What is Python?

• Interpreter based language, allows the execution of one instruction at a time
• Support extensive basic data types: numbers , strings, lists, dictionaries
• Variables can be strongly typed as well as dynamic typed
• Support OOP concepts – class/ inheritance / obj/ module/ namespace
• cleaner exception handling
• Automatic memory management

Where to use?

• Data Science
• Web Development
• Image Processing
• Game Development
• Rapid Development tool (CPython , Jython)

Python Syntax

print( 'id:' , 1)

• Multiple lines code uses \ as the indicator

if 100 > 99 and \
    200 <= 300 and \
    True != False: \
        print('Yay')

• use ; to separate multiple statements in a single line
• expressions inside () [] {} are not required to use \ for multilines

Comment

# Single Line comment

```
multiline comment
```

How to use user input?

name = input() print(name)

age = input() type(age)

#%% md

Python Variables

• For memory optimization, Python stores the same value variable to the same address when value changes, it changes the address

#%%

type(10)

Data Types

Scalar types:

• int
• float
• complex (5+2j)
• bool (True False)
• None

Sequence Type

• String
• List
• Tuple (not necessary in same data type)

Mapping type

• Dictionary

Set Type

• Set
• frozenset

Mutable vs Immutable

• Number / Strings / Tuples are Immutable
• List / Dictionary / Set are mutable

Number

int

• _ can be deliminators x = 1_232_234

int(‘5.5’) = 5

float

float(‘ -5’) = -5.0

Arithmetic Operators

• • • / %

** exponent // floor division

pow(2,3) = 8 abs(-5) = 5

String

String are immutable sequence data type.

‘this is a string’ “this is also a string” ‘'’this is still a string ‘’’ (multiple line string) “"”well, string””” (multiple line string)

#%%

s = ‘i love you’ len(s) # 10

retrieve the length of the string

#%%

greet = ‘hello’ greet[0] greet[4]

#%% md

str() class

str(100) -> ‘100’ str(True) -> ‘True’ str1 = r’Welcome to ' Python '’ -> Welcome to ' Python ' r before string is escaper’s escaper

• operators:
  • • [] [ : ] in not in
• Methods: str.capitalize() str.index(“hello”) === first occurance of hello str.count(“p”) == count the num of P str.isdigit() str.split(‘ ‘)

#%% md

List

List Class

myList = list('Hello')

nums = list({1:'one',2:'two'})
print(nums)  #  [ 1 , 2 ]

nums=list((10, 20, 30))
print(nums) #  [10, 20, 30]

nums=list({100, 200, 300})
print(nums) #  [100,200,300]

names = ["Jeff", "Bill", "Steve", "Mohan"]
del names[0] # removes item at index 0
print("After del names[0]: ", names)

names.remove("Bill") # removes "Bill"
print("After names.remove(\"Bill\"): ", names)

print(names.pop(0)) # return and removes item at index 0
print("After names.pop(0): ", names)

names.pop() # return removes item at last index
print("After names.pop(): ", names)

del names # removes entire list object
print(names)

list.append() list.extend() list.clear() list.copy() list.remove(element) list.insert(index, value) list.reverse() => inplace reverse list.sort(key=, reverse=True) key : a function extract the key from each element studentList.sort(key=lambda s: s.name)

#%% md

Tuples

immutable ordered collection, preserves the order of the elements cannot declared by a single element s = (‘sss’) ==> string must be s = (‘sss’ , ) ===> tuple

names = (‘mike’, ‘luke’, ‘rick’) names = ‘mike’, ‘luke’, ‘rick’

a,b,c = names

update of the tuple element is not supported —> error del names

class tuple()

tuple(‘hello’)

len(tuple) max(tuple) min(tuple)

#%% md

Set

Mutable hashable distinct object collections. Unordered: cannot get access by index

Set cannot add a list set cannot add a set

set({1:’one’, 2:’Two’}) dic -> set, conserve the keys ==> {1,2}

set.add(element) set.update(another_set) set.remove(element)

s1.union(s2) s1|s2 s1.intersection(s2) s1 & s2 s1.difference(s2) s1 - s2 s1.symmetric_difference(s2) s1^s2

#%% md

Dictionary

Collection type: not an ordered sequence, contain Key: value pairs. list cannot be the key of dict. since list is mutable

• Accessing a Dict: dict.get(“France”) dict[“France”] = “Paris” del dict[“France”] del dict

  for k in dict.keys()

  for v in dict.values()

  for k, v in dict.items()

#%% md

Built-in Modules

Collections

1. namedtuple() collections.namedtuple(‘student’, [name, age, marks]])

   s1 = student(“Imran”, 21, 98)

   s1.name s1[0]

2. collection.OrderedDict()

3. deque() q = collection.deque([10,20,30,40]) q.appendleft(0) q.append(50) q.pop() ==> 50 q.popleft() ==> 0

Random

random.random() ==> 0.0~1.0
random.randint(1,100)
 random.randrange(1,10) ---> select one from the range
 random.randrange(0,100,10) ---> 20
 random.choice([1,2,3,5,7,3,2])
 random.shuffle([2,4,67,3,2,2,4])

#%% md

Package

1. the python interpreter recognizes a folder as a package by its ‘init.py’ file

2. init.py exposes specified resources from its modules to be imported empty= all exported

   from .functions (child file) import average, power(function name)

#%% md

main

there’s an default variable in python file: name which indicates the current scope of the execution. if the program is running at the basic level of python , aka direct run in the console, the scope is main however, if a script is imported as a module into other scripts. now thwe scope is the package name itself. it’s not main

Main is not the entry function like Java.

#%% md

Iterator

iter() Iterators are implicitly used whenever we deal with collections of Lists, tuple or strings When we use

for i in list:
    #do something
    pass

internally, it uses iter(list) L1 = [1,2,3] it = iter(L1)

it.next() (equals to) next(it)

throw stopIteration error when nothing more to iterate.

#%% md

Generator

other than iterator, this is the thing you can create your own iterator this is a special type of function, it doesn’t return a single value. it returns an iterator object with a sequence of values.

#%% md

Map Function

map is a built in function the map function calls the specified function for each item of an iterable it will return an iterable object,

map(function, iterable)

def square(x):
    return x**2

numbers = [1,2,3,4,5]
sqrList = map(square, numbers)

# map with lambda
sqrlist = map(lambda x : x*x, [1,2,3,4])

#%% md

Filter Function

filter(function, iterable ) –> filter object (iterable)

#%% md

##Reduce Function

import functools
def mult(x,y):
    print("x=",x," y=",y)
    return x*y

fact=functools.reduce(mult, range(1, 4))
print ('Factorial of 3: ', fact)

#%% md

Class

Python is a OO language. Every element in python is an Object of a class. Number / String / dict /

Define a class

class person: ‘'’doc string: this is empty person class’’’ pass

p1 = person()

class Person:
    count = 0 # class attribute
    def __init__(self):
       self.name = "unknown"
       self.age = 0
    def displayInfo(self):
       print(self.name, self.age)

init(self) is a constructor

private:

class employee: def init(self, name, sal): self.__name=name # private attribute self.__salary=sal # private attribute

>>> e1=employee("Bill",10000)
>>> e1._employee__salary
10000
>>> e1._employee__salary=20000
>>> e1._employee__salary
20000

• Previous
  Data Structures - Primitive Types
• Next
  进程间/线程间通讯